1. Field of the Invention:
The present invention relates to a drive system for turbochargers with motor-generators mounted on their rotatable shafts, and more particularly to a drive system for a multistage turbocharger assembly with series-connected turbines and compressors.
2. Description of Prior Art:
There are known turbine-driven turbochargers in which exhaust gases emitted from an engine are led to an exhaust turbine to rotate the turbine and a compressor coupled thereto at a high speed for supercharging the engine. Japanese Laid-Open Patent Publication No. 60-43152 discloses a system for recovering and feeding any remaining exhaust energy back to the shaft of the engine after the exhaust energy from the engine has been recovered by the exhaust turbine to drive the compressor. The disclosed system has a rotary electric machine and an intake air compressor which are mounted on the rotatable shaft of the exhaust turbine. The exhaust energy which is recovered as rotational energy by the exhaust turbine is used to rotate the compressor for supercharging the engine. At the same time, the rotary electric machine operates as an electric generator to generate electric energy which is supplied to an electric motor coupled to the shaft of the engine. The motor coupled to the engine shaft is rotated to assist in rotating the engine shaft, thereby feeding the exhaust energy recovered by the exhaust turbine back to the engine shaft.
When the engine rotates at a low speed and hence the exhaust energy is small, however, the above system can neither generate electric energy nor supercharge the engine. To avoid this shortcoming, a system has been proposed a system in which when the engine rotates at a low speed, the rotary electric machine mounted on the rotatable shaft of the exhaust turbine is operated as a motor to rotate the compressor through the rotatable shaft for supercharging the engine (see Japanese Laid-Open Patent Publication No. 60-195329).
When the engine rotates at a low speed and under a high load, the rotary electric machine mounted in the turbocharger is driven as an electric motor to assist in operating the compressor for thereby increasing the boost pressure under which air is charged into the engine. Therefore, the temperature of the engine at the end of its compression stroke rises. In a thermally insulated engine with combustion chamber inner walls made of thermally insulating material, the temperature at the compression stroke end is higher than that of conventional engines. Since the temperature in the combustion chambers of the engine is high, fuel supplied into the combustion chambers tends to start being ignited faster than a preset timing .
The system disclosed in Japanese Laid-Open Patent Publication No. 60-195329 does not vary the timing for injecting fuel into the engine. Therefore, when the engine undergoes a high load at which the temperature in the combustion chambers is high, fuel supplied into the combustion chambers is not combusted under normal conditions. As a result, the engine output power is lowered, and pollutants contained in exhaust gases are increased.